1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to subsea oil and gas drilling equipment. More particularly, the present technology relates to accumulator valves for use in subsea oil and gas drilling hydraulic circuits.
2. Discussion of the Background
Blowout preventers (BOPS) are important safety components in subsea well drilling operations. Typically, a BOP is attached to a wellhead at the sea floor, and provides a bore through which the drill string can pass from the top of the BOP down through the bottom and into the well. The BOP is equipped with BOP rams, which are located on opposing sides of the bore and are designed to close across the bore if needed. Some rams are sealing rams, which seal around the drill pipe to close in the annulus of the well. Other rams are shearing rams, and are designed to shear the drill pipe and anything else in the bore, to completely close in the bore. The BOP and its rams provide an effective barrier against dangerous pressure surges that may develop in a well.
In order to operate the BOP rams, hydraulics are typically used to drive the rams from an open to a closed position. Hydraulic fluid is applied to the rams via a fluid conduit that connects the rams to a fluid reservoir or accumulator. A valve or series of valves in the fluid conduit controls the fluid flow through the conduit, which in turn determines the hydraulic pressure applied to the rams. The forces needed to drive the BOP rams can be large, as the equipment is heavy, and much force may be required to shear the steel drill string and other components in the bore. Accordingly, if it becomes necessary for an operator to fire the rams and close the BOP, a significant amount of hydraulic pressure is applied to close the rams.
Because the hydraulic pressure needed to close the rams is high, the corresponding rate of hydraulic fluid flow through the conduit is also high. Accordingly, when the supply valve opens to allow fluid flow to drive the rams, the change in velocity of fluid at the rams can be large and sudden. Similarly, when the supply valve closes at the end of the function, the fluid flow is suddenly stopped. These sudden changes in velocity lead to pressure spikes in the fluid at the opening and closing of the supply valve, which pressure spikes are typically referred to in the industry as hydraulic shock, or water hammer. Water hammer can cause significant damage to components on the BOP.
In addition, after maintenance or during initial start-up of BOP equipment, hydraulic lines can require air to be purged from the system. This is typically done by cycling the equipment to fill the lines. During air purging, water hammer can be induced by the rapid hydraulic velocities involved with such a fill and purge.